1. Field of the Invention
The present invention relates to a moire eliminating filter used in the case where filtering processing for eliminating a grid image is carried out on image data representing a subject image having a stripe pattern (moire) superposed on the image and corresponding to a grid used in photographing, and also to an image processing method and apparatus using the filter.
2. Description of the Related Art
Radiation image recording reproducing systems using stimulable phosphon which emits light, upon exposure to stimulating rays such as visible light, in accordance with radiation energy stored therein originated from a radiation (such as X rays, xcex1 rays, xcex2 rays, xcex3 rays, electron rays, and ultraviolet rays) having been irradiated thereon have been known (see Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-164645, and 55(1980)-116340, for example). A radiation image recording reproducing system records a radiation image of a subject such as a human body on a stimulable phosphon sheet and the sheet is scanned with stimulating rays such as a laser beam in order to cause the sheet to emit light. The emitted light is read photoelectrically by reading means such as a photomultiplier and an image signal is obtained thereby. Based on the image signal, the radiation image of the subject is output by the radiation image recording reproducing system, as a visible image on a recording medium such as a photosensitive material or on a CRT display.
When a radiation image of a subject is recorded on a stimulable phosphon sheet or the like, a grid composed of alternately placed, in a fine pitch such as 4 lines/mm, lead or the like which is not penetrable by radiation and aluminum or wood or the like which is penetrable by radiation is placed between the subject and the sheet in some cases so that radiation scattered by the subject is not irradiated on the sheet. Since radiation scattered by a subject is rarely irradiated on a sheet when the grid is used at the time of photographing, contrast of a radiation image of the subject can be improved. However, a fine stripe pattern (moire) corresponding to the grid is also recorded on the sheet together with the subject image, and a reproduced image becomes indistinct.
For this reason, a method of obtaining a distinct image with reduced moire by carrying out Fourier-transform followed by inverse Fourier transform on image data to eliminate frequency data corresponding to a grid pattern, or by carrying out filtering processing to eliminate a spacial frequency component corresponding to a grid pattern has been proposed (Japanese Unexamined Patent Publication Nos. 3(1991)-12785 and 3(1991)-114039, for example). In the case where a grid pitch is 4.0 lines/mm for example, a stripe pattern appears around a spacial frequency band of 4.0 cycles/mm. Therefore, in the above method, the stripe pattern is eliminated by applying filtering processing using a filter to eliminate or reduce a response of this frequency band.
The grid pitch described above is not limited to 4.0 lines/mm, and 3.4 lines/mm is used in some cases. When such a grid of 3.4 lines/mm is used, a stripe pattern should appear in a spacial frequency band of 3.4 cycles/mm. However, since the filter used in Japanese Unexamined Patent Publication No. 3(1991)-12785 and the like is to eliminate or reduce a response of only a specific spacial frequency, a stripe pattern caused by a grid of a different pitch cannot be eliminated thereby. As a result, a reproduced image has moire and is not distinct. For example, in the case where a stripe pattern is eliminated by using a filter having a characteristic shown in FIG. 7 while an image is being reduced by xc2xd, a response of a spacial frequency of 4.0 cycles/mm caused by a grid having a pitch of 4.0 lines/mm can be reduced. On the other hand, a response of a spacial frequency of 3.4 cycles/mm caused by a grid having a pitch of 3.4 lines/mm cannot be reduced. As a result, a stripe pattern caused by the 3.4 line/mm pitch grid appears in a reproduced image. Furthermore, in the case where a stripe pattern is eliminated by using a filter having a characteristic shown in FIG. 8 while an image is being reduced to ⅔, the response of the spacial frequency of 4.0 cycles/mm caused by the grid having the pitch of 4.0 lines/mm can be reduced. On the other hand, the response of the spacial frequency of 3.4 cycles/mm caused by the grid having the pitch of 3.4 lines/mm cannot be reduced.
In this case, filtering processing using a filter for eliminating or reducing a response of a spacial frequency corresponding to a pitch of a grid to be used is possible. However, at the time of filtering processing, it is difficult to identify what pitch the grid used at the time of photographing has. Therefore, filters of a plurality of kinds are necessary, and memory space for storing the filters becomes large.
The present invention has been conceived based on consideration of the above problems. An object of the present invention is to provide a moire eliminating filter and an image processing method and apparatus using the filter for carrying out filtering processing on image data in order to cause a stripe pattern generated by a grid to become inconspicuous even when a grid pitch varies.
A moire eliminating filter of the present invention reduces a response of a spacial frequency component of a stripe pattern in image data obtained by reading an image including the stripe pattern corresponding to a grid of n line/mm pitch used at the time of photographing the image, and the filter reduces the response of a spacial frequency component equal to or more than 97% of the spacial frequency component corresponding to the grid pitch to equal to or less than 5%, preferably to equal to or less than 2%.
As the grid pitch, 3.4 lines/mm, 4.0 lines/mm, and the like are used, and moire caused by these grids appears in spacial frequencies of 3.4 cycles/mm and 4.0 cycles/mm, for example. However, since the grid may not have been set parallel to a sheet or attached to a sheet incompletely, the spacial frequency of moire can shift slightly to the lower frequency side. Therefore, in the present invention, the spacial frequency component corresponding to the grid pitch is shifted to the lower frequency side by approximately 3%, and a response of a spacial frequency component (for example, 3.3 cycles/mm) of equal to or more than 97% of the spacial frequency component corresponding to moire (for example, 3.4 cycles/mm) is reduced to equal to or less than 5%.
Meanwhile, in the case where frequency data of the spacial frequency corresponding to moire are filtered by using Fourier transform, the response of the spacial frequency can be 0. However, this operation is time consuming. In order to carry out filtering processing quickly, filtering processing in real space, not in frequency space, is used. However, in the real space, the degree of a real space filter is finite and a response of a specific spacial frequency cannot be 0. Therefore, in the present invention, the response of spacial frequency components of equal to or more than 97% of the spacial frequency component corresponding to moire is reduced to equal to or less than 5%, preferably to equal to or less than 2%.
The image processing method of the present invention carries out filtering processing by using the moire eliminating filter of the present invention.
An image processing apparatus of the present invention comprises filtering means for carrying out filtering processing using the moire eliminating filter of the present invention.
According to the moire eliminating filter of the present invention, since the response of the spacial frequency components of equal to or more than 97% of the spacial frequency component corresponding to a grid pitch is reduced to equal to or less than 5%, a stripe pattern caused by a grid of any pitch can be reduced after image data have been filtering-processed. Therefore, by subjecting image data to filtering processing using the filter of the present invention, a reproduced image with an inconspicuous stripe pattern can be obtained.